Recently, flexible and plane speakers in futuristic applications have generated much interest. Application in areas such as 3C (computer, communication and consumer electronics), smart windows, smart curtains, automobile audio and toys have been actively discussed. However, some novel sound generating techniques are not completely suitable for futuristic audio systems needs, such as energy-saving, flexible structure and design freedom of shape, etc. Hence, concerns with electret flexible speaker improvements are growing and have been perfected to complete the idea.
A traditional type of electret actuators has been studied since the 1970s. Taking a typical structure, an electret-based diaphragm is placed beside perforated electrode layers and separated by a set of spacers. The speaker operates in membrane vibration mode, interaction between the externally applied voltage and space charge of an electret induced vibration on the diaphragm is done by varying the electrostatic force which in turn induced acoustic waves to be radiated. Results show that the inherent advantages included a simple and compact construction, better efficiency, and excellent high-frequency response. Hence, from Coulomb's law, to obtain a high efficiency electret speaker, the electret diaphragm should possess high charge storage and a light mass. By effectively enhancing the charge density, we can obtain an efficient device.
To obtain a high performance electret speaker, the electret diaphragm should possess good charge storage capability and a light mass. By effectively enhancing the charge density, we can obtain an efficient device. Porous polytetrafluoroethylene (PTFE) films are recognized as great electret material with excellent charge-storage capabilities. However, despite the advantages and benefits of porous PTFE, disadvantages include characteristics such as a difficulty to adhere to an electrode layer layer, a medium charge storage stability at high porosity thin-film, a low elastic modulus, and easy plastic deformation at low stress. These disadvantages have hindered the further development of flexible electret speakers. Some studies have attempted to improve the properties of porous PTFE which adopt coating and lamination methods to form a composite material. However, the resulting composite material becomes less conformable than desired. Although difficult to achieve, the ideal properties for a good electret diaphragm include features such as low cost, a good adhesion between the electrode layer and the PTFE, and a light mass.